[PATCH] Make cstddef / cstdarg more robust (PR 69881)

[Jonathan Wakely]

On 23/02/16 07:15 +0000, Bernd Edlinger wrote:
>as described in the PR 69881 it happens quite often that cstddef is
>called with __need_size_t because we still support gmp-4.3.2 which
>is installed by contrib/download_prerequisites.  This causes a kind
>of undefined behavior.  It is just by chance that this does not cause
>the gcc-6 boot-strap to fail, but it causes gcc-4.9 bootstrap to fail
>when the gcc-6 version of cstddef is around.  So it looks like a
>regression, because the new cstddef is more fragile than before.

Is it? cstddef hasn't been changed since 2013-06-11. cstdarg hasn't
been changed since 2011-01-30.

What made them more fragile?